Sewing machines have been known which are capable of executing a so called free motion sewing operation. In a free motion sewing operation, a feed dog for longitudinally feeding a workpiece cloth is inactivated within a bed and a presser foot for applying pressure on the workpiece cloth releases its pressure exerted on the workpiece cloth. The user is allowed to manually feed the workpiece cloth freely under such state. The free motion sewing operation will end up in poor looking stitches if the stitches are not formed at a constant stitch pitch. However, forming stitches at a constant stitch pitch through manual feeding of the workpiece cloth has been a difficult task for inexperienced users.
To address such difficulties, a sewing machine capable of executing the free motion sewing operation at a constant stitch pitch is proposed, for example, in JP-2002-292175 A hereinafter referred to as patent publication 1. The disclosed sewing machine is provided with a distance measuring element which measures the distance of travel of fed workpiece cloth and a needle speed changing element that changes operating speed of the sewing needle based on the measurement. In another example of a sewing machine disclosed in JP 2008-79998 A hereinafter referred to as patent publication 2, an imaging element and a feed amount regulator are provided. Under the disclosed configuration, the feed amount of workpiece cloth is calculated based on the image data captured by the imaging element. The feed amount regulator compares the calculated feed amount with a predetermined stitch pitch and limits the feed amount based on the comparison.
However, the sewing machine disclosed in patent publication 1 requires additional features such as the distance measuring element and the needle speed changing element, whereas the sewing machine disclosed in patent publication 2 requires additional features such as the imaging element and the feed amount regulator. Both sewing machines disadvantageously require complicated configurations.